1. Field of the Invention
The invention concerns a vehicle transmission, in particular for utility and agricultural vehicles, in which the speed changing arrangement (for example, a multi-speed gearbox and a range gearbox) is followed by a central differential which divides the gearbox output power between the front axle and the rear axle. Agricultural tractors of this type frequently have their front and rear axles fitted with unequally large tires.
2. Description of the Related Art
Despite varied proposals, current four wheel drives for agricultural tractors are not entirely satisfactory. With a rigid drive to the front and rear axles with zero overrun for the front axle, field operation results in propulsion forces that are too low and negative propulsion forces when operating around curves.
One way to reduce this effect is to increase the overrun to 2% or more, but there are still problems in operation around curves. As the steering angle increases, the front wheel on the inside of the curve increasingly loses its overrun and the slip changes from positive to negative. This means that the wheel is retarded and no longer propels the vehicle. In operation around curves this means the turning radius actually increases when the mechanical front wheel drive (MFWD) is engaged. In operation on the road the result is increased tire wear and, in the field, shearing of the grass shoulder. This is particularly apparent in the tight turns approaching the headland of a field.
One possible solution to this problem is to disable the front wheel drive for on the road operation. Unfortunately, this in turn means that the front axle cannot be utilized for propulsion or engine braking.
In theory, this disadvantage could be partially avoided by automatically engaging the front wheel drive during braking when the gearbox is in the road operation range. However, this might result in uncontrollable operating conditions.
Further attempts at solving the problem include adding steps in the gear ratios for the front wheel drive, while disabling the outer front wheel using a non-slip differential or the like. This has the disadvantages that it strains the transmission to the front wheel, may cause difficulty in controlling shifting between pulling and braking, and it limits drive to only three of the four wheels of the tractor.
EP-B1-0 112 421 and EP-B1-0 113 490 have disclosed vehicle transmissions of the aforementioned type with central differentials, by which the engine torque can be divided in a predetermined ratio between the front axle and the rear axle. This avoids strains between front and rear axles.
It is known to use spur gear planetary gearsets as central differential gears, but they require considerable radial space with their planets, carriers and ring gears. Since their gear ratio depends largely on the conditions of the installation, the central differentials limit the design possibilities for the vehicle transmission. If the central differential is applied in connection with a synchronized gearbox, for example, its radial dimensions must be compatible with the established shaft spacing of the synchronized gearbox. This requirement is not always met easily, particularly with compact gearboxes with high power density.